Emissive flat-panel display devices are widely used in conjunction with computing devices and, in particular, with portable devices. These displays are often used in public areas with significant ambient illumination and are viewed from a wide variety of angles. Such devices are also under development as area illumination devices or lamps.
Light emitting diodes (LED) incorporating thin films of light-emitting materials have many advantages in a flat-panel display device and are useful in optical systems. Such films can comprise either, or both, organic and inorganic materials. U.S. Pat. No. 6,384,529 issued May 7, 2002 to Tang et al. shows an OLED color display that includes an array of OLED light-emitting elements (sub-pixels). Light is emitted from a sub-pixel when a current is passed through an organic material, the frequency of the light depends upon the nature of the organic material that is used. The organic materials are placed upon a substrate, between electrodes, with an encapsulating cover layer or plate. In such a display, light can be emitted through the substrate (a bottom emitter), or through the encapsulating cover (a top emitter), or both. The emitted light is usually approximately Lambertian, that is, the brightness of the device is independent of the viewing angle. Because LED devices employ high-optical-index emissive materials, a large fraction (e.g. greater than 50%) of the emitted light is trapped in the device due to total internal reflection and thus reduces the device efficiency. Inorganic materials, for example, may include phosphorescent crystals or quantum dots. Other thin films of organic or inorganic materials may also be employed to control charge injection, charge transport, or charge blocking to the light-emitting-thin-film materials, and are known in the art.
Optical cavity structures are known to affect the light emitted from an LED device. When a pronounced optical effect is seen and when formed in thin films with at least one semi-transparent or semi-reflective layer, such optical cavity structures are known as microcavities or optical microcavities. However, in any LED device employing transmissive and reflective electrodes with an emitting layer formed between the electrodes that emit light into air, an optical cavity structure producing optical interference effects will be present. The optical cavity will have an optical cavity length that can include the electrodes. An optical cavity length between two electrodes is the sum of the thickness times the refractive index of the layers between the two electrodes.
When formed in LED devices, different color light-emitting organic materials are pattern-wise deposited over a substrate between a reflective electrode and a semi-transparent electrode, for example a thin silver electrode. Optical microcavities are tuned to a desired peak wavelength of light, typically corresponding to the color of light emitted by the patterned light-emitting materials. U.S. Pat. No. 6,680,570 by Roitman et al. describes an organic light-emitting device with improved color control employing spacer layers to form an optical cavity. FIG. 9 illustrates such a prior-art, active-matrix, bottom-emitting optical microcavity device employing a substrate 10 with active-matrix thin-film components 30, planarization structures 32 and 34, and a semitransparent electrode 16. Patterned organic materials 14R, 14G, and 14B providing red, green, and blue light emission are deposited in a light-emitting layer(s) 14 with a reflective electrode 12 formed over the light-emitting layer(s) 14. Optical spacers 26R, 26G, and 26B are employed to form optical cavities 60, 62, and 64 tuned to the desired peak wavelengths of red, green, and blue light, respectively to emit red light 70, green light 72, and blue light 74. A cover 20 can be employed to protect and encapsulate the device.
While such designs are useful, they require a patterned organic-material deposition technology (for example, vacuum deposition through metal shadow-masks) that is difficult to scale to large substrates. Moreover, optical-microcavity devices typically suffer from unacceptable angular color dependence. It is also known to employ a color filter with an optical cavity structure, for example as taught in U.S. Pat. No. 7,180,238 by Winters. However, while useful, such an approach does not improve the manufacturability of the device and provides inadequate ambient contrast ratio under some illumination conditions. Moreover, the color filters absorb light emitted from the light-emitting layer, thereby reducing device efficiency.
U.S. Pat. No. 5,554,911 entitled “LIGHT-EMITTING ELEMENTS” by Nakayama et al. describes a multi-color light-emitting element having at least two optical cavity structures with respectively different optical lengths determining their emission wavelengths. Each optical cavity structure includes an organic material as a light-emitting region, which may be a single film of uniform thickness in the element. U.S. Pat. No. 6,861,800 entitled, “TUNED MICROCAVITY COLOR OLED DISPLAY” by Tyan et al. describes a microcavity OLED device having an array of pixels divided into at least two different color pixel sets, each color pixel set emitting a different predetermined color light over a common substrate, wherein each pixel in the array includes a metallic bottom-electrode layer disposed over the substrate and a separate semitransparent metallic electrode layer spaced from the metallic bottom-electrode layer. The material for the semitransparent metallic electrode layer includes Ag, Au, or alloys thereof The thickness of the semitransparent metallic electrode layer, the combined thickness of the organic layers and the transparent conductive phase-layer, and also the placement of the light-emitting layer are selected so that each pixel in the display forms a tuned microcavity OLED device having emission output efficiency above that of a comparable OLED device without the microcavity. U.S. Pat. No. 5,949,187 by Xu et al. describes an OLED with a first microcavity including a first transparent spacer and a first mirror stack positioned on the first spacer to reflect light back into the OLED and to define an optical length of the first microcavity. The optical length of the first microcavity is such that light emitted from the first microcavity has a first spectrum. A second microcavity includes a second transparent spacer positioned adjacent the first microcavity and a second mirror stack positioned on the second spacer reflects light toward the second microcavity and defines an optical length of the second microcavity. The optical length of the second microcavity is such that light emitted from the second microcavity has a second spectrum. Additional microcavities can be placed in the structure to further enhance and alter the light spectrum. Such designs, however, can have increased manufacturing costs, lower light output than desired, and reflectance larger than may be desired, as well as significant color change at different viewing angles, owing to the change in the effective optical path length for light traveling at angles to the normal.
US Patent Publication No. 2006/0066228 entitled, “REDUCING OR ELIMINATING COLOR CHANGE FOR MICROCAVITY OLED DEVICES” by Antoniadis et al. discloses a microcavity OLED device that reduces or eliminates color change at different viewing angles. The OLED device can be, for example, an OLED display or an OLED light source used for area illumination. This OLED device includes a multi-layer mirror on a substrate, and each of the layers is comprised of a non-absorbing material. The OLED device also includes a first electrode on the multi-layered first mirror, and the first electrode is substantially transparent. An emissive layer is on the first electrode. A second electrode is on the emissive layer, and the second electrode is substantially reflective and functions as a mirror. The multi-layer mirror and the second electrode form a microcavity. On a front surface of the substrate is a light modulation thin film. The light modulation thin film can be any one of: a cut-off color filter, a band-pass color filter, a brightness enhancing film, a microstructure that attenuates an emission spectrum at an angle at which there is a perceived color change, or a microstructure that redistributes wavelengths so the outputted emission spectrums have the same perceived color. Again such designs can have increased manufacturing costs due to patterned deposition processes. Also, significant light can be absorbed by the color filters thereby reducing efficiency.
One approach to overcoming material deposition problems on large substrates is to employ a single emissive layer, for example, a white-light emitter, together with color filters for forming a full-color display, as is taught in U.S. Pat. No. 6,987,355 entitled, “STACKED OLED DISPLAY HAVING IMPROVED EFFICIENCY” by Cok. However, the use of color filters substantially reduces the efficiency of the device. It is also known to employ a white sub-pixel that does not include a color filter, for example as taught in U.S. Pat. No. 6,919,681 entitled, “COLOR OLED DISPLAY WITH IMPROVED POWER EFFICIENCY” by Cok et al. However, this disclosure does not address angular color issues or the large amount of trapped light.
A microcavity OLED device can also produce white-light emission. U.S. Pat. No. 7,268,485 entitled, “WHITE-EMITTING MICROCAVITY OLED DEVICE” by Tyan et al. describes a white-light emitting OLED apparatus including a microcavity OLED device and a light-integrating element, wherein the microcavity OLED device has a white-light emitting organic EL element and the microcavity OLED device is configured to have angular-dependent narrow-band emission, and the light-integrating element integrates the angular-dependent narrow-band emission from different angles from the microcavity OLED device to form white-light emission. However, this can reduce the color gamut of the device. U.S. Pat. No. 7,030,553 entitled, “OLED DEVICE HAVING MICROCAVITY GAMUT SUB-PIXELS AND A WITHIN GAMUT SUB-PIXEL” by Winters et al. discloses an example of a microcavity device. This disclosure describes an OLED device including an array of light-emitting pixels, each pixel including sub-pixels having organic layers including at least one emissive layer that produces light and spaced electrodes. There are at least three gamut sub-pixels that produce colors that define a color gamut and at least one sub-pixel that produces light within the color gamut produced by the gamut sub-pixels. At least one of the gamut sub-pixels includes a reflector and a semitransparent reflector, which function to form a microcavity. However, this design employs a patterned semi-transparent electrode to form the white subpixel that can be difficult to manufacture in a top-emitting format. Moreover, angular color change is not addressed in the patent.
There still remains a need, therefore, for an improved light-emitting structure that overcomes shortcomings in the prior art and that improves the angular color performance of an LED device.